(1) Field of the Invention
This invention relates to a polyamide fiber which is used for making a polyamide fiber having an improved moisture absorption and/or antistatic property, and for making a fabric having an improved moisture absorption, antistatic property and feeling.
Fabrics such as woven and knitted fabrics made from the polyamide fiber of the invention are valuable especially as clothes for summer wear and sport wear, for which a high hygroscopicity is required, and as clothes for underwear and lining cloth, for which antistatic property is required.
(2) Description of the Related Art
Conventional polyamide fibers (hereinafter may be called as "nylon fibers") have characteristics such as high tenacity, softness and pile stability against compression, and hence, have been widely used for stockings, carpets, sport wear and underwear. Nevertheless, clothes of polyamide fibers are not satisfactory in moisture absorption, although their moisture absorption is better than those of polyester fibers and acrylic fibers. Namely. it is said that sport wear and summer wear get readily humid and an improvement in comfortableness is eagerly desired. Further, polyamide fibers have a poor antistatic property, and hence, underwear stimulates the skin and, when taken off, it makes a sparkling sound due to electrical discharge. Thus, an improvement in antistatic property also is eagerly desired.
With regard to feeling of clothes, a weight-reduction treatment using an alkali is popularly carried out on polyester woven and knitted fabrics to enhance the bounce resilience, suppleness and drapability and to provide a variety of polyester fiber fabrics of different feelings. In sharp contrast, an alkali-treatment cannot be employed in polyamide woven and knitted fabrics. Although an acid treatment can be theoretically applied to polyamide fabrics, the acid treatment is of little or no practical use because problems arise such that an acid is usually toxic to a certain extent and causes corrosion of apparatuses. Therefore, there is no practical method of giving a variety of feelings to polyamide woven and knitted fabrics.
An attempt has heretofore been made to improve the moisture absorption of polyamide fibers by imparting a hygroscopicity-enhancing agent to the fiber surfaces by an after-treatment. However, the application of a hygroscopicity-enhancing agent has problems such that the fastness to washing is poor and, when the amount of the hygroscopicity-enhancing agent is increased to improve hygroscopicity, the undesirable waxy hand increases.
Further, a proposal has been made to graft-copolymerizing an acrylic acid onto a polyamide and then neutralizing the thus-obtained graft-copolymer to introduce a sodium carboxylate group (--COONa) into the copolymer. This proposal has problems such that a high percentage of graft copolymerization and thus a desired high hygroscopicity are difficult to obtain, or, even though a high degree of graft copolymerization can be obtained, the waxy hand increases to a considerable extent.
Another proposal has been made to render a polyamide itself hydrophilic, for example, by copolymerizing polyamide-forming monomers with a polyoxyalkylene glycol or other hydrophilic ingredients (Textile Research Journal 55, 325-333 [1985]). A high copolymerization ratio of the hydrophilic ingredient is required for a desired high hygroscopicity, but it leads to reduction of mechanical properties and light resistance and appearance of waxy hand.
Thus, attempts for imparting polyamide fibers a good hygroscopicity solely by a chemical modification of a polyamide have been unsuccessful. Still another proposal has been made wherein a polyamide is combined with a high hygroscopicity-giving polymer so that the desired properties of the two polymers manifest themselves. For example, a core-sheath type conjugate fiber comprised of a highly hygroscopic polyamide core and a lowly hygroscopic polyamide sheath is described in Unexamined Japanese Patent Publication No. H3-213519. This conjugate fiber is costly because a complicated manufacturing apparatus must be used. Further, it is difficult to keep the cross-sectional shape constant over a long period of time in the fiber-making step, and hence, a dyeing speck and streaks are liable to appear in woven fabrics and knitted fabrics made therefrom. Further, the two polyamides for the composite fiber usually have different melting points and the melt spinning thereof must be carried out at a temperature higher than the melting point of the polyamide having a higher melting point than that of the other polyamide. At the high temperature melt spinning, the polyamide of a lower melting point is liable to be thermally degraded and the spinnability is lowered.
As a further proposal of imparting a good hygroscopicity to a polyamide, a physical modification process has been proposed wherein a soluble ingredient is incorporated in a polyamide, the mixed polyamide is spun into a fiber and then the soluble ingredient is extracted with a water or another solvent from the fiber to increase the moisture-absorbing surface area of the fiber whereby a polyamide fiber exhibiting an enhanced moisture absorption and rate of moisture absorption is obtained. However, if the amount of the soluble ingredient is small, the hygroscopicity of the fiber obtained is insufficient. In contrast, if the amount of the soluble ingredient is large, the mechanical properties of the fiber are lowered and, when clothes thereof are worn, they are subject to whitening and fibrillation. Thus, the hygroscopicity and the mechanical properties are incompatible with each other.
To impart an antistatic property to a polyamide fiber, a proposal has been made wherein an antistatic agent comprising a hydrophilic ingredient such as polyoxyalkylene glycol and an ionic ingredient such as an alkylsulfonic acid metal salt, a benzenesulfonic acid metal salt or a higher fatty acid metal salt is incorporated in a polyamide fiber. A large amount of the antistatic agent must be added for the antistatic property of a desired level. But, the incorporation of a large amount of the antistatic agent leads to lowering of the spinnability and the mechanical properties of fiber, and, when worn, the clothes are subject to whitening and fibrillation. Thus, the antistatic property is incompatible with the spinnability and the mechanical properties.